Experiments in open chest dogs with normal hearts will simulate loss of contractility and increased compliance of part of the left ventricle. This will be accomplished by cannulating the ventricle through the apex. Blood will be removed and replaced once per heart cycle by a servo-controlled syringe pump. The left ventricular pressure will be monitored and will allow the electronic circuit to prescribe a given previously specified volume-pressure relation for the additional compliance which simulates the presence of the myocardial infarction. Thus the decreased pumping performance of the heart, measured as cardiac ejection fraction and rate of pressure rise during isovolumic systole, may be investigated as the three parameters of cardiac function, muscle contractility, passive wall compliance, and size of the "infarcted" area are changed independently.